Stephen, there is sample variance. When spending so much money on a lens it is best to buy it from a dealer who will allow you to exchange it within a short time period if you are not satisfied with the sample you buy.
I own one of these lenses - this website published a test comparison I did between the 24~105L and the 28~135 IS (http://www.luminous-landscape.com/reviews/lenses/24vs28.shtml
). The lens I tested was a sample from the original batch. Canon recalled all those lenses because they said there was excessive flare (which I never noticed as being excessive). The lens they gave me in exchange was less crisp than the "defective" one I returned. So, much to Canon's surprise, I promptly retrieved my "defective" lens and I have been extremely pleased with it ever since.
I don't do formal tests with resolution line charts. I'm only interested in what real-word photographs will look like as a print. There is a brick community center protected by link-fencing up the street from where I live. The sun hits it at different angles as the day progresses and it provides all the detail, texture and contrast I need to judge whether a lens is satisfactory. I do a matrix of shots at wide, 50mm and telephoto at wide-open, mid-range and f/11 f-stops.
The most challenging performance test for these zoom lenses is wide angle at wide aperture. That is where they are at their weakest. But since only a small proportion of your photos are likely to be made with those settings, it is best to judge performance accross the matrix.
Of course, testing this way you will never know whether you have the BEST this lens can deliver (you would need to test many lenses under identical conditions to judge that), but at least you will know whether the image quality is satisfactory relative to your expectations.
A note about post-processing. There are two schools of thought. One is to judge the RAW files with no sharpening to see what the lens does unaided - but biased by the influence of the sensor's anti-aliasing filter. The other is to sharpen the RAW file as you would normally, because this is how you will use the product. I think both approaches tell you something, but the latter is the bottom line.